Very High Idle ISV Problem

[Christopher Flanner]

Listers,

My 90tq decided to act up on the way to dinner Friday night, challenging me
to parallel park with a 3000 rpm idle.  The engine is an MC from an '86
5ktq.  The throttle valve is fully closed and the idle returns to normal if
the ISV hose is pinched shut.  I opened the ISV and it wasn't sticking open
but I cleaned it anyway.  It seems that the pressure differential over the
ISV is sufficient to open it.  While it's idling at 3000 rpm the vacuum is
about 4 inH2O higher than regular idle at over 22 inH2O.

I don't have a clear picture of the idle air bypass layout in my head but is
it possible that the screw-adjusted bypass is blocked causing vacuum to open
the ISV valve?  I tried to blow and suck through the ISV (smart, I know) and
it is very much closed but when the engine runs/idles there is definitely
air passing through it even when it's disconnected (electrically).  I won't
mention that I got an oily piece of junk caught in my throat when I tried to
perform CPR on the ISV.  Oops, too late.

My ISV isn't receiving any signal but I don't think that's my problem.  If
the connnector is disconnected on an ISV it should stay closed, right?  I
went through the Bentley ISV diagnostics (which aren't for an MC but most
details are similar) and everything checked out leading to the
recommendation of replacing the control unit.  Once again, I don't think
that's related to my problem since having no signal usually leaves the valve
closed.

Summary:  The ISV moves freely and is normally closed but when the engine
runs, it is apparently open, even though it is disconnected, and causes the
engine to idle at 3000 rpm.

What's going on?????




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